Ethylene-propylene-nonconjugated diolefin terpolymers are characterized by an absence of polymer backbone unsaturation which renders these materials extremely resistant to oxidation and ozone. Rubber compounders have attempted to use these low unsaturation ethylene-propylene-nonconjugated diolefin terpolymers in compositions with highly unsaturated elastomeric materials such as natural rubber, styrene-butadiene rubber, polybutadiene rubber and acrylonitrile-butadiene rubber in order that the terpolymers' oxidation and ozone resistance may be imparted to the highly unsaturated rubbers. However their efforts have not been successful due to the fact that the terpolymers will not compatibly cocure with highly unsaturated natural rubbers or diene rubbers in blends such as those that are desirably made in the preparation of stocks for passenger car tires, truck tires, aeroplane tires and heavy duty off-the-road tires.
A technique by which the cure compatibility of butyl rubber with natural rubber and styrene-butadiene rubber has been greatly improved has been by the halogenation of butyl rubber. Although the ethylene-propylene-nonconjugated diolefin terpolymers that are presently commercially available are quite different in structure when compared with butyl rubber, it has also been found that halogenation of these terpolymers likewise results in a marked improvement in their cure compatibility with the highly unsaturated rubbers. Unfortunately, the halogenated ethylene-propylene-nonconjugated diolefin terpolymers in which the nonconjugated diolefin is a monomer such as dicyclopentadiene, 5-ethylidene-2-norbornene, 5-vinyl-2-norbornene or 1,4-hexadiene are found to have poor stability as evidenced by undesirable increases in viscosity during storage or when exposed to elevated temperatures associated with processing or finishing treatments. In addition, known ethylene-propylene-nonconjugated diolefin terpolymer halogenation processes frequently cause an undesirable increase in gel content which seriously detracts from the processability of the material. Thus there is a recognized need in the rubber industry for a halogenated ethylene-propylene-nonconjugated diolefin terpolymer that exhibits a stability on storage and exposure to elevated temperatures superior to that shown by halogenated ethylene propylene-nonconjugated diolefin terpolymers which have been previously synthesized.